This invention relates to the application of protective coatings to the interior seams of cans and more particularly to the application of protective coatings to the interior of the welded overlapped seam or the butt weld seam of a three-piece metal can.
Metal cans are generally made by either of one of two processes. One process, the two-piece can process, involves forming a drawn cup from a flat sheet of metal by a blanking process and then further forming the cup to a can configuration by an ironing process. The other process, the three-piece process, involves forming a cylindrical can body from a sheet of metal and then attaching two lids to the opposite ends of the body. The present invention is concerned only with the application of protective coatings to three-piece cans.
In the manufacture of three-piece cans, the cylindrical can bodies are formed by wrapping a sheet of metal around a so-called stubhorn. The ends of the sheet are either butted or overlapped and secured together by a welded seam, a soldered seam or a cemented seam. The interior of the seam is then coated with a protective coating, the function of which is to protect the contents of the can against the metal contaminants. In the application of this coating, continuity is extremely critical because any pinholes, cracks or imperfections in the integrity of the coating will generally render the can unsuitable for most applications.
After application, the coating is cured by heating. This curing process is applied only to the area of the can seam. Therefore, any coating material applied to the interior of the can which is not upon the seam area will not be cured. Uncured coating material will also contaminate the contents of a can.
The present invention is primarily concerned with applying this continuous coating onto welded can seams. Welded can seams tend to have more irregular projections than soldered can seams and usually require a coating thickness of about six milligrams of dried resin per square inch. A method of coating welded can seams is disclosed in Boone U.S. Pat. No. 4,337,281. The disclosure of this patent is incorporated herein by reference. The Boone '281 patent discloses coating a welded can seam by applying a flow coat of higher solids coating material onto the welded can seam. A flow coat generally refers to a non-atomized liquid curtain of coating material. A higher solids coating is generally 20 to 40 percent solids or more. This is by far the best method of applying a stripe of coating material onto the welded seam of a can. However, since the coating is applied immediately after the welding operation and the welding operation increases the temperature of the seam to about 1300.degree. F., the coating material does not always cover the can seam as desired. The heat of the seam either causes the coating to boil, leaving a pitted surface, or causes the coating to run off the seam before the coating is cured at the heating station.
Attempts have been made to cool the cans prior to the coating step, but due to the high speed of the operation, they have been unsuccessful. For example, one attempt has been to spray the seam area with a jet of air to cool the seam area. Another method is to spray the outside of the seam area with water. The can bodies simply move too fast to be cooled by the air jet and coating with water has been very unsuccessful. The water per se is a contaminant to the coating. If any water is on the seam area, the coating material will cover it. When the coated can advances to the curing station, the elevated temperature causes the water to boil, pitting the surface of the coating.
Accordingly, it is an object of the present invention to provide a method and apparatus for cooling the seam of a can to facilitate the application of a stripe of coating material. Furthermore, it is an object of the present invention to accomplish this without substantially increasing the cost of the coating equipment.
It is further an object of the present invention to apply a continuous stripe of coating material onto a welded seam of a can wherein the stripe is uniform in thickness, continuous and not excessively thick.
These objectives are accomplished by spraying a cooling material onto the seam of the can, and subsequently applying a flow coat of material onto the seam of the can. The cooling material or coolant is a material which is compatible (i.e., mixes or dissolves) with the coating material. Preferably, the coolant is the same material as the coating material, the solvent used in the coating material or the coating material with a lower solids content.
In the preferred embodiment, these objectives are accomplished by spraying a coating material onto the seam of a can from a nozzle having two orifices. The first orifice emits an atomized cooling spray of coating material, and the second orifice emits an unatomized or flow coat of material.